1. Field of the Invention
The invention relates to an apparatus for medical diagnosis and/or therapy which includes drive means for driving at least one movable part of the apparatus and a detection device for detecting the presence of an object that is in contact with the moving part of the apparatus, which detection device includes:
sensor means for detecting an instantaneous force exerted on the movable part,
comparison means for comparing said force with a value expected for said force that is available in the apparatus,
which comparison means are arranged to produce an alarm signal when a predetermined difference between said force and said expected value is exceeded, and
response means for initiating a reaction by the drive means in response to the alarm signal.
2. Description of the Related Art
An apparatus of this kind is known from U.S. Pat. No. 5,570,770.
Generally speaking, an apparatus for medical diagnosis and/or therapy may be provided with a radiation emitter and a radiation receiver. An example in this respect is formed by a medical X-ray apparatus provided with an X-ray source and an X-ray receiver which is usually referred to as an image intensifier. These two elements are arranged at some distance from one another, the patient to be examined or treated being positioned between the X-ray source and the image intensifier. The X-ray source and the image intensifier are positioned relative to the body of the patient such that an image can be formed of the desired cross-section of the body (the object). The orientation and the position of such apparatus can often be adjusted by means of drive means in the form of a motor drive. Generally speaking, in the context of the present invention the term object is to be understood to mean the body of a patient to be examined or another body to be examined, the body or a part of the body of a person operating the apparatus, parts of the apparatus itself (for example, the patient table) or of neighboring apparatus, or other obstacles that could enter the path of movement of the parts of the apparatus.
Such apparatus are often provided with a so-called C-arm, that is, a circular support which is rotatable in its own plane (that is, about an axis extending perpendicularly to the plane in which the C-arm is situated) by way of a trackway, the plane of said C-arm itself being rotatable about an axis situated in said plane. In many cases there is also a large number of other possibilities for displacement.
During use of the apparatus it is important that a movable part, for example the image intensifier, comes close to the object to be examined in order to achieve the desired clarity of the image. The image intensifier has a comparatively large front surface for receiving the X-rays and any point on this front surface or on its circumference could come into contact with the object to be examined. Such a collision may occur in any direction of movement of the image intensifier. This is undesirable and, therefore, an apparatus of this kind is provided with a detection device for detecting the presence of an object situated in the vicinity of or in contact with the movable part of the apparatus.
It is important to include such a detection device notably in motor-driven apparatus. When contact is detected between the movable part of the apparatus and an object, the movement of said (part of the) apparatus can be stopped so as to minimize the seriousness of the consequences of a collision.
The cited U.S. Pat. No. 5,570,770 discloses a medical X-ray apparatus that is provided with an electrical detection device for detecting the presence of an object that is in contact with a movable part of the apparatus. The sensor means in such an apparatus are arranged to measure the current and/or the power taken up by the driving motor. These quantities form an indication of the instantaneous force exerted on the movable part; this instantaneous value can be compared with a value expected for said force that is stored in the apparatus. When the difference between the instantaneous value and the expected value exceeds a predetermined threshold value, it is assumed that the movement of the movable part is impeded by an object and hence that a collision takes place. Consequently, an alarm signal is generated and in response thereto the driving motor is made to react; for example, the motor can be stopped and braked or only stopped; it can also be made to perform a retracting motion after stopping.
Because the sensor means in the known apparatus measure the current and/or power taken up by the driving motor, the determination of the instantaneous force exerted on the movable part is inaccurate and not very well reproducible. On the one hand, this may lead to the conclusion that a collision occurs even though there is no such collision, or on the other hand, to late detection of an actual collision occurring. Evidently, both situations are undesirable for medical equipment.
It is an object of the invention to provide an apparatus of the kind set forth in which a state of collision can be determined in a more accurate and better reproducible manner. To this end, the apparatus according to the invention is characterized in that the sensor means include a force sensor which is arranged in such a position that it detects directly the instantaneous force exerted on the movable part. The invention is based on the recognition of the fact that said imperfections are due to the fact that a large number of intermediate parts for transmission and driving are present between the movable part of the apparatus and its drive. Each of said intermediate parts introduces friction, mass inertia, play and elastic deformation; moreover, such phenomena are highly dependent on the instantaneous position and acceleration of the movable parts and on the state of ageing of the apparatus. When the instantaneous force exerted on the movable part is determined without such an intermediate arrangement of these components, the effect thereof is eliminated.
The forces exerted on the movable part of the apparatus in a preferred embodiment are transferred to the apparatus via guide means that bear on one of the other parts of the apparatus, the force sensor being arranged in or directly on said guide means. The guide means notably include the bearing arrangement of a drive spindle for driving the movable part. In this embodiment an as direct as possible coupling is realized between the force sensor and the movable part.
In conformity with a further embodiment of the invention the apparatus is provided with state sensing means for determining the positional and/or motional state of the apparatus and the comparison means in the apparatus have available a data set of expected values, said data being a function of the positional and/or motional state of the apparatus. As a result of these steps a variety of variable effects exerted on the force to be determined can be taken into account, for example the effect of the force of gravity for different states of the apparatus. In this context the term xe2x80x9cdifferent statesxe2x80x9d is to be understood to mean all parameters defining the state of the relevant movable part of the apparatus. In the case of an image intensifier supported by a C-arm, therefore, this may be the angular rotation of the C-arm (in its own plane and about an axis situated in said plane) and the height adjustment of the image intensifier relative to its guide. These steps also enable other variable effects exerted on the force to be determined to be taken into account, for example, the acceleration of the various parts of the apparatus, again in different states of the apparatus. The assembly of such parameters constitutes the positional and/or motional state of the apparatus. Said state sensing means detect the positional state and/or motional state of the apparatus and the set of data thus obtained (that is, the instantaneous value of the positional state and/or motional state of the apparatus) then acts as a variable for the selection of the instantaneous expected value from the data set of expected values.
A further embodiment of the apparatus in accordance with the invention is provided with state sensing means for determining the positional and/or motional state of the apparatus, the comparison means in the apparatus being provided with processor means for calculating and presenting said expected value on the basis of a model of the apparatus and in dependence on the positional and/or motional state of the apparatus. These steps also enable the effects exerted on the force to be determined to be taken into account, said state sensing means determining the positional and/or motional state of the apparatus. The set of data presented by the state sensing means in this embodiment is applied to a processor whose software contains a model of the apparatus, that is, the equations necessary to calculate the expected value for the relevant force in dependence on said set of data. This embodiment offers the advantage that effects that vary in time, for example, frictional forces or play that change due to wear or elastic deformation that changes due to a reduced rigidity of components, can be readily adapted by modifying the relevant numerical values in the software. These values are, for example, the numerical values of the friction coefficients, the magnitude of the play and the elasticity moduli.